This invention provides improvements to the apparatus and methods disclosed in PCT/US02/12443, published application WO 02/086386, the disclosure of which is hereby incorporated in its entirety by reference.
The flaring or assisted open combustion of undesired process by-product streams is commonly used to oxidize and convert toxic gases and vapors to their less harmful combustion products for release into the environment. A mixture of the undesired product and a fuel are directed to the base of the flare stack to form a feedstream that rises to the flare tip or stack outlet where the mixture is ignited in the combustion zone to form the flare or flame. The efficient and complete combustion of the mixture is not always achieved. When the process is not properly managed, smoke is also produced by this process. Smoke can be an indicator that the combustion process is incomplete, and that the toxic or otherwise undesired process materials have not been converted to less harmful forms. Smoke is also a visible constituent of air pollution, and its elimination or reduction is a consistent operational goal.
In order to reduce smoke production, the installation of auxiliary pressurized air and steam systems in conjunction with flaring stacks is well known in the prior art. The low-pressure air assist system uses forced air to provide the air and fuel mixing required for smokeless operation. A fan, commonly installed in the bottom of the flare stack, provides the combustion air required. Steam assisted flare systems use a steam ring and nozzles to inject steam into the combustion zone at the flare tip where air, steam and fuel gas are mixed together to produce a smokeless flame. In some systems of the prior art, a concentric barrier or shield surrounds the flare tip or outlet in order to channel atmospheric air into a rising mass that is drawn to the gases emitted from the flaring stack barrel.
Steam and low-pressure air assists for flaring are in common use because both systems are considered by the art to be generally effective and relatively economical as compared to alternative means for disposing of the undesired by-products.
However, both of these prior art systems have various drawbacks and deficiencies. The low-pressure air assists requires a significant capital expenditure for at least one fan that must be dedicated to the flare stack. Steam assist systems typically require sophisticated control devices, and have relatively high utility requirements and maintenance/replacement schedules. Continuous operation imposes a rigorous maintenance schedule and even a back-up system in case of a breakdown or a requirement for major repairs.
An improvement to these prior art systems, as disclosed in WO 02/086386 is a plurality of high pressure air jet nozzles positioned on a manifold located between a concentric shield and the exterior of the flare stack outlet. The adjacent surface of the shield is perforated to enhance the flow of atmospheric air into the space between the shield and the stack. In practice, this construction was found to be effective in eliminating or substantially reducing smoke. However, the related structure at the top of the stack was exposed to extremely high-temperature combustion gas resulting in a shortened useful life for the equipment.
Based upon operating experience with the apparatus and methods of the prior art as disclosed in WO 02/086386, it has been found that the enhanced combustion of the feedstream gas components was achieved along with the suppression of smoke. However, the increased concentration of heat in the turbulent gases was found to have shortened the life of the metal components employed to control and direct the gaseous flow of the feedstream and the induced ambient air flow, as well as the high and low pressure air jets and associated piping. Thus, the need exists to provide an apparatus and method for improved flaring that will extend the useful operating life of the fabricated metal components at the flaring tip.
It is therefore an object of this invention to provide improved apparatus and methods of operation of a stack that will avoid the concentration of high temperature turbulent gases in the proximity of the tip components.
Another object of the invention is to provide means for controlling the mass of pressurized air to assure adequate mixing with the feedstream and the complete combustion of the undesired chemical component and fuel based upon predetermined actual stoichiometric requirements.
Yet another object of the invention is to operate the flaring stack so that the combustion zone is elevated above the shield and other related tip components in order to minimize their exposure to the burning gases at their highest temperature.
It is another principal object of the present invention to provide an apparatus and method for enhancing the complete combustion of flare gases that is highly effective in promoting the efficient and complete combustion of the fuel and undesired chemicals without smoke, that requires minimal maintenance, and that is adaptable to the variation in day-to-day operating conditions that may be expected in industrial plant operations.
Another object of the invention is to provide a method and apparatus that is readily adapted for use with existing flaring stacks without significantly modifying the existing stack barrel and feedstream component delivery system.
The terms flaring stack and flare stack are used interchangeably in this description. As used herein atmospheric air means the ambient air surrounding the stack and is distinguished from air pressurized delivered via high or low pressure conduits and/or discharged from nozzles. Sources of pressurized air delivered to the nozzles should be free of debris to avoid interfering with the operation of the nozzles.